You can't edit the file through FTP because the file isn't saved in your home directory. You have to log into the shell. However, you can get started with FTP:
1) Create a single line file called cron.txt that contains:
30 * * * * /full/path/and/command
This will run the command at half past every hour. Your software may tell you when the command needs to run.
2) FTP the file into your home directory.
3) Log into your shell account using PuTTY to get to the command line.
4) Type the following command to enter your file into the cron system:
5) To verify that it's in, type:
(that's the letter "ell") This should echo back what was in your cron.txt file.